Thousands of children world-wide exhibit cranio-facial defects, and the treatment of these deformities is challenging. Various surgical techniques are used to treat these problems. However, clinical advancements in facial bone reconstruction have been limited. Presently, we still depend largely on autogenous or autologous bone grafts for the reconstruction of the non-stress bearing as well as the stress-bearing facial skeleton. These grafts are difficult to contour, are available only in limited quantities, and can undergo significant graft resorption. Synthetic materials such as hydroxyapatite or carbon-based polymers have proved useful in aesthetic augmentation of the facial skeleton but they frequently lack the structural stability necessary for major head and neck reconstruction. Furthermore, these synthetics have proved to be uniformly inadequate for the reconstruction of stress-bearing bone such as the mandible (1,2,3,4,5,6). In addition, complications occur subsequent to surgery, such as devitalisation of teeth, partial or complete loss of an osteotomized segment and bone graft failure (7,8,9,10). Distraction of an osteogenesis has been used in recent years to overcome the problems with the surgical techniques discussed above. In this application, the term "distraction osteogenesis" is the method of growing new bone by stretching pre-existing bone. This technique, which was first described by Ilizarov et. al. (11), involves surgically fracturing the bone with minimal disruption of its periosteal and endosteal envelope. After the surgery, a waiting period of 7 to 10 days allows new bone cells to populate the area. At the end of the waiting period, the bone on each side of the fracture site is stretched by various mechanical means at a rate of 1.0 mm per day (11-16).
Currently, distraction osteogenesis has been clinically applied on a widespread basis within orthopedic surgery. It has been used for the replacement of segmental defects, the treatment of fracture non-unions and the elongation of shortened extremities. This technique has also been used within orthopedic surgery for the correction of traumatic congenital deformities in both the long bones of the extremities and the smaller bones within the hands and feet. (17-20)
Although distraction osteogenesis has been applied to orthopedic problems on a widespread basis, it has been used for facial skeletal reconstruction on an experimental basis only. Clinical experience in orthopedic surgery indicates that bone created by distraction osteogenesis is structurally stable and that it should be able to withstand the shear and torsional stresses that the mandible is subjected to during mastication. The main disadvantage of using distraction osteogenesis for facial skeletal reconstruction is that the orthopaedic mechanical devices are too large to be applied within the craniofacial systems.
In orthopedics, numerous pin and ringed fixators are available for limb lengthening, including those taught by Wagner, De Bastiani (Orthofix), Ilizarov, Monticelli-Spinelli and Acefisher. All of these mechanical devices are external to the skin (20,21,22,23). The problems reported in the literature with respect to these devices are as follows (24):
1. External mechanical devices are large and cumbersome and interfere with the patient's daily lifestyle. PA1 2. Pin loosening and pin-tract infection results in the development of soft tissue inflammation/infection at the external site and this leads to bone infection. PA1 3. Neurovascular injuries occur due to pin placement. PA1 1. Due to the fact that the activation mechanisms of these devices are outside the skin, the pins cut the skin during the activation procedure, resulting in the formation of scar tissue. PA1 2. The pins used in these devices are unstable and weak. There have been a number of reports of pin loosening, resulting in relapse. PA1 3. Pin instability increases as the amount of distraction to be carried out increases. Pin instability also increases as the length of the healing period increases. PA1 4. Because the devices are extra-oral, the pin-tract infection that has been reported in orthopedic cases has also been observed with the devices used in the craniofacial system. PA1 5. The extra-oral and cumbersome nature of these devices along with the formation of scar tissue has negative effects on patient co-operation and psychology.
The first attempt to apply the principles of distraction osteogenesis within the craniofacial system to lengthen the mandibular body was by Snyder et al. (36) in 1973. An external (outside the skin) screw-driven device attached with bicortical pins was used.
In 1990, Karp (32) and his colleagues used an external (outside the skin) bone-lengthening device at the mandibular expansion site. The device was comprised of a standard external fixator with two double-pin holders and a bolt at one end that allowed calibrated distraction or compression of the pins (Howmedica Corp., Rutherford, N.J.).
In 1992, McCarthy (30) and his co-workers used this external (outside the skin) device to lengthen the mandible on children with congenital growth defects.
Also in 1992, Luhr et al. described an extendable bone plate to fix the fracture of small bones (U.S. Pat. No. 5,129,903). The Luhr device exhibits a lack of sectional modulus to support compressive load and the self-locking feature is questionable. Testing with respect to technical specifications and animal or clinical studies has not been disclosed.
In 1995 Molina and Monasterio (40) in Mexico City reported on clinical applications of external pin distraction osteogenesis in the mandible in 106 patients. The mean age of patients was 8 years of age and surgical technique utilized was intra oral corticotomy with placement of two percutaneous pins. The distraction appliance was activated extra orally. The mean follow up was 19 months and x-rays were taken at selected intervals. In their patient group all patients received bite back orthodontic therapy post operatively.
These authors report temporomandibular joint pain in the unaffected side with evidence of bony remodelling of the non-treated temporomandibular joint. Skin scars were noted for all patients but the authors claim no infection, fractures, but did relate skin inflammation and three patients requiring antibiotic therapy. These authors did accomplish distraction osteogenesis in the mandible with no reported relapse. There is poor documentation regarding long term radiographic follow up. There is no comment on neuro sensory changes in the inferior alveolar nerve or discussion regarding devitalization of tooth roots and tooth buds due to pin placement. Additionally all patients received orthodontic bit block therapy which may have contributed significantly to long term stability of the distracted bony segments.
In 1995 McCarthy et al. (41) reported on experimental development of an intra oral mandibular distraction appliance. This was applied to ten mongrel dogs with distraction osteogenesis. Pins were placed percutaneously but stabilized to an intra oral appliance. The appliance was bulky and crude and had minimal stabilization with two pins per segment of the mandible. Additionally the appliance did not have the multi directional capacity of adjustment in many vectors required for distraction osteogenesis of the mandible.
In 1996 Chin and Toth (42) reported on distraction osteogenesis utilizing internal devices in patients. An internal device was fastened to the mandible via screws and was distracted via trans oral activation. This technique was utilized in three patients. Premature consolidation of bone was noted in two patients and one patient had significant relapse. The authors comment that improved stability of attachment on the device to bone was needed. This appliance also did not have multi directional capabilities of controlling the vector of distraction of the mandible. The authors also report of breakage of one of their expansion screws which required further surgery to remove and replace this. Bilateral appliance was also used to advance the mid face and maxilla but was activated through a percutaneous screw which would leave an unsightly scar.
U.S. Pat. No. 5,364,396 to Robinson et al discloses an implantable distraction device which is proposed to permit gradual bone distraction between osteotomically separated bone sections. This device is made up of welded components and permits direct control of the bony segments. It is proposed to have a low profile, although its size and other specifications are not disclosed. It is not telescopic. The device has one force vector and there is no disclosure with respect to testing.
The Robinson et al device is implanted subcutaneously, however, actuation is extraorally via a percutaneous (through unbroken skin) port projecting outward through the skin. The percutaneous site would be subject to pin-tract infection and scar formation as the actuation port would project through the skin during the actuation and retention periods. As parts of Robinson's device are welded together, the device lacks the compressive strength needed for an effective distractor. Furthermore, as Robinson's device is not telescopic, it lacks a double displacement path and results in a larger device.
Tschakaloff (39) and his colleagues used a subperiosteal calvarial distractor for the correction of craniosynostosis in rabbits in 1994 (Medicorn Micro Systems, Tuttligen, Germany).
All the extra-oral devices used to date to conduct distraction osteogenesis is the craniofacial system have the following problems, as reported in the literature: (25-37).
Thus, a need exists for a distraction device that is stable, easy to activate and completely tissue-covered.